I don't think you can use the jack on the back for anything, you need to use a seperate sender. Luckily, the V+ box uses standard RC5 infra red signals, so pretty much any remote sender will work.
The thing about them not working with Virgin is in regard to the older boxes which used IRDA standards and wouldn't work. If you want some more info, have a look at
www.redremote.co.uk
I use a Powermid XL made by Marmitek (now Eurovox I think). You can pick them up for about £20 - £25.